This invention relates generally to audio signal processing and is particularly directed to an arrangement for compensating for nonlinear operating characteristics of a microphone.
The intensity of sound is defined as the average rate of sound energy transmitted in a specified direction through a unit area normal to this direction at the point considered. Sound intensity is usually measured in decibels (dB's), in which case it is known as the intensity level and is equal to 10 times the logarithm (to the base 10) of the ratio of the sound intensity to a reference level. The sound itself is caused by pressure changes, or vibrations, characterized as having intensity (loudness) and pitch (frequency). The frequency range over which these sound-generating pressure vibrations are audible is termed the audio spectrum which covers approximately 20 Hz-20kHz.
A microphone is an electroacoustic transducer which responds to sound waves and delivers essentially equivalent electric waves in the form of signals for recording, amplification, broadcasting, measurement, etc. Ideally the microphone will have a uniform pressure response for all frequencies and directions. However, in reality the operating characteristics of microphones vary with frequency and are nonlinear over the audio spectrum. This characteristic degrades the audio signal even before it is subjected to other variations in the signal processing circuitry coupled to the microphone.
In the past, the technique used for most real time spectrum analyzers has been to provide a calibrated input to the microphone during its assembly by means of a test fixture. The microphone response is then manually adjusted over each of the operating frequency bands to provide a substantially uniform signal output. This procedure is time consuming and therefore expensive and is subject to human error. In addition, the microphone and its associated audio signal processing circuitry are now inseparable requiring the microphone to be recalibrated if any changes are made to the integrated combination. Finally, this type of microphone calibration arrangement is subject to drift with time and use particularly where the calibration is accomplished by means of manually adjustable potentiometers as is frequently the case.
The present invention is intended to overcome the aforementioned problems and limitations of the prior art by providing a read only memory (ROM) compensated microphone wherein the ROM is programmed to provide a response which is precisely complementary to the response of the microphone such that when the two components are combined the result is a flat frequency response across the entire audio spectrum. The integrated ROM and microphone combination thus provides a linear operating characteristic over this frequency range in the audio signal processing device in which it is used.